Observation of the Falling Motion of Early Snowflakes.

Part II: On the Variation of Falling Velocity

Abstract

The variations of vertical and horizontal velocity of early snowflakes, which are composed of two to six crystals, were analysed by a stereo-photogrammetric method. The following main results were obtained:
1) The standard deviation of variation in vertical velocity is usually smaller than 2cm s-1 (3 to 5% of the mean vertical velocity). On the other hand, the standard deviation of variation in horizontal velocity is considerably larger (1 to 8cm s-1) and increases with the mean horizontal velocity. These results are independent of the main type of component snow crystals. Accordingly, it seems likely that the variation of horizontal velocity plays an important role in the aggregation process of snowflakes.
2) The mean horizontal velocity can be estimated from the non-dimensional amplitude of the spiral or rotational falling motion of snowflakes.
3) The non-dimensional amplitude is inversely related to the non-dimensional frequency of spiral or rotational motion.
4) The non-dimensional frequency can be estimated from the Reynolds number with respect to the mean vertical velocity of snowflakes.